If mobs (spiders, splumonkeys, etc.) wear Polly Roger's Hat, Polly Roger will spawn and collect an item. Polly Roger cannot successfully return the item to the mob, and will keep holding onto it (flying away with the item if the mob is killed.) I noticed with splumonkeys that there's a constant loop of landing and takeoff while near the wearer (didn't see this with friendly Webber spider.)
Let mob that can wear hat obtain a Polly Roger's Hat.